The transmitters released by afferents to the serotonin (5HT) neurons of the median raphe nucleus were examined. We injected into that nucleus agonists and antagonists of the neurotransmitter GABA, and then measured 5HT turnover and/or 5-hydroxyindoleacetic acid content in the hippocampus. Turnover of 5HT was estimated with the aid of high pressure liquid chromatography (HPLC) by measuring the probenecid-induced accumulation of 5-hydroxyindoleacetic acid. We found that intra raphe injection of the GABA agonist muscimol decreased hippocampal 5HT turnover. Conversely, the GABA antagonists, picrotoxin and bicuculline increased GABA turnover. The data show that raphe contains GABA receptors; the stimulation of these receptors has a tonic inhibitory control over the firing of raphe 5HT neurons.